Herpes simplex virus type 2 (HSV-2) is the leading cause of genital herpes. HSV-2 is most often transmitted by sexual contact, and infection with the virus typically leads to recurring outbreaks of lesions on the genitals and perianal regions, combined with shedding of virus into the genital tract. Viral shedding can also occur in the absence of lesions or other symptoms. HSV-2 also establishes latency in sensory ganglia. HSV-2 infection causes physical discomfort and psychosexual morbidity in affected patients, and introduces additional health risks. In particular, patients infected with HSV-2 are at increased risk for contracting HIV, and pregnant mothers infected with HSV-2 can vertically transmit HSV-2 to their fetuses. In immunocompromised individuals or in neonates, HSV-2 infections can be fatal. Currently, there is no cure for HSV-2 infection.
HSV-2 infection is widespread, with one study estimating that nearly 20% of the population worldwide is infected (Looker et al., 2008, Bulletin of the World Health Organization, October 2008, 86(10)). More women than men are infected, and the prevalence of the disease increases with age. High numbers of adolescents diagnosed with HSV-2 indicate that the prevalence across the population will continue to rise, as HSV-2 infection is lifelong.
Treatment options for HSV-2 symptoms are limited. Antiviral therapy, using compounds such as famciclovir, valaciclovir, or aciclovir, limits the duration of symptoms and, in some cases, speeds healing of lesions and reduces incidence of viral shedding. Antiviral drugs are not curative, however, and do not prevent recurrence of outbreaks or clear the virus completely. In addition, use of antiviral drugs requires patients to recognize symptoms of HSV-2 infection, then obtain a confirmative diagnosis, and ultimately, comply with the antiviral regimen. These requirements may be untenable in regions of the world where antiviral drugs are not readily available. In addition, patients are often unaware that they are infected, either because they do not present symptoms, or because the symptoms of the initial infection subside, suggesting recovery from the disease.
To address the medical and social problems associated with HSV-2, it is highly desirable to develop pharmaceutical compositions to inhibit or counteract infection by HSV-2. An effective composition may be used to elicit an enhanced immune response against HSV-2, thereby preventing initial infection, blocking the ability of the virus to establish latency in sensory ganglia, eliminating recurrence of outbreaks, and/or preventing viral shedding. The immune system is known to mount a defense against HSV-2, as evidenced by recurrent infections which manifest with fewer, less intense symptoms and decreased frequency over time.
While the ultimate goal of an HSV vaccine would be long-lasting protection from viral infection, the suppression of disease symptoms would also provide significant health benefits. One of the current goals for either a prophylactic or therapeutic vaccine is to reduce clinical episodes and viral shedding from primary and latent infections. Three categories of prophylactic vaccines have been tested in clinical trials with disappointing results: i) whole virus, ii) protein subunit, and iii) gene-based subunit vaccines (Stanberry et al., Clinical Infect. Dis., 30(3):549-566, 2000). In the 1970s a number of killed virus vaccines were explored, none of which were efficacious. More recently an attenuated HSV was found to be poorly immunogenic. Subunit vaccines based on two recombinant glycoproteins have been clinically evaluated in combination with different adjuvant formulations. One developed by Chiron contains truncated forms of both glycoprotein D (gD2) and glycoprotein B (gB2) of HSV-2, purified from transfected Chinese Hamster Ovary (CHO) cells and formulated with adjuvants alum and MF59. Another developed by Glaxo-Smithkline (GSK) contains a truncated gD2 formulated with adjuvants alum and 3-O-deacylated monophosphoryl lipid A (MPL). Both vaccines were immunogenic and well tolerated in phase I/II trials. However in phase III analyses, the Chiron vaccine showed no overall efficacy against HSV-2 seroconversion and work was discontinued. The GSK vaccine showed significant efficacy (73-74%) in HSV-1, HSV-2 seronegative women volunteers but no efficacy in men.
While even limited vaccine efficacy would beneficially impact HSV sufferers, these trials are testing only a small number of vaccine possibilities. This is because the vaccine discovery has not been systematic. Pursuance of a whole-virus vaccine assumes that presentation of the pathogen itself to the immune system will generate optimal immunity. Indeed the breadth and duration of immune responses to whole pathogen vaccines historically have been better than subunit vaccines. However, pathogenicity of the vaccine strain must be considered. Subunit vaccines, to date, have been selected for vaccine testing based on their assumed importance in disease pathogenesis and immunogenicity during infection. These approaches have identified one candidate against HSV with limited efficacy in some but no efficacy in other formulations. Thus, new and improved methodologies for herpesvirus vaccine discovery are needed to protect against herpes diseases.